Cheapest company to have an iPhone with?

Unlimited text,
least amount of calling minutes to make it cheaper if possible
and the least amount of data if cheaper.

Added (1). Not just a phone. An iPhone

Check virgin mobile usa website. $30 for unlimited internet, 300 min talk and unlimited text…

Straight talk is 45$ a month for unlimited everything.
Gophone is 15$ a month (you can buy the 1000 txt package)
T-Mobile is as low as 10$ a month

Those are all prepaid networks.

-edit-
You can have prepaid networks on an iPhone, I currently have straight talk on an iPhone.

Today? Probably T-Mobile USA.

They have a Monthly4G prepaid (no-contract) plan for $30 per month: 100 mins of calls, unlimited text, unlimited data (the first 5 gigabytes at 4G speeds). Additional voice minutes (over 100) are billed at $0.10/min which is very competitive to other pay-as-you-go per-minute rates.

Cellular service providers periodically change their offerings. It might be a different company six months from now. Six or seven months ago, it was probably AT&T GoPhone. Then they change their Pay As You Go plans and removed data packages.

I'm using the aforementioned T-Mobile Monthly4G $30 plan with my factory-unlocked iPhone 4S. I previously used Straight Talk's $45 unlimited plan. Before that, I used AT&T GoPhone's Pay As You Go per-minute plan with data packages.

Straight Talk $45
Unlimited Talk/Text/Web